1. Field of the Invention
The present invention relates to a semiconductor memory device with a refresh trigger.
2. Description of the Related Art
The biggest features of a non-volatile memory cell used for a NAND flash memory are as follows. The non-volatile memory cell has a polysilicon floating gate whose surroundings are covered with an insulating film. A memory cell threshold voltage is varied in the following manner. Specifically, a voltage (control voltage) applied to a control gate nearest to a floating gate (FG) is controlled. A charge, that is, electrons are injected from a substrate to a floating gate via FN tunneling. Moreover, a voltage (erase voltage) applied to well is controlled to pull a charge out of the floating gate to vary the threshold voltage of the memory. If the foregoing variation is not wider than a fixed margin, the function as the memory device is lost.
Conversely, advances in scale reduction are made to reduce a bit price, and thereby, the following problem arises. The variation margin becomes narrow due to interference between cells and FG fringe capacitance; as a result, scale reduction is hindered. Moreover, the following phenomenon (IPD leak) makes control of variation difficult. According to the phenomenon, electrons tunnel through an inter-poly dielectric (IPD) held between a floating gate (FG) and a control gate (CG). For example, if the IPD leak occurs when electrons are injected from the substrate to the floating gate, the following problem arises. Namely, the threshold voltage of the memory cell is not established as a target value.
In order to solve the foregoing problems, the following method has been proposed (JP-A 2006-310662 [KOKAI)). According to the method, both ends of an inter-poly-insulating film held between the floating gate and the control gate are replaced with metal films.
However, the foregoing method includes using new substance and technique that is not used for a conventional non-volatile memory. For this reason, a large amount of costs are spent for the development of element forming technology. Resulting from affinity with process conditions peculiar to the non-volatile memory, it is very difficult to develop the foregoing new substance and new technology.
For this reason, it is desired to realize a non-volatile memory which can secure storage contents even if data retention time becomes short resulting from thinning of a tunnel film.